Fasting Evidence

Protocol

5:2 and alternate-day fasting

These produce the largest weight losses of the fasting protocols, for the least surprising reason: they create the largest deficits.

Updated 2 min read 10 citations Evidence strength 4/5

What the protocols actually involve

The two "modified fasting" protocols
ProtocolFasting daysIntake on those daysOther days
5:2Two non-consecutive days per weekRoughly a quarter of normal intakeUnrestricted
Alternate-day (modified)Every other dayRoughly a quarter of normal intakeUnrestricted
Alternate-day (true)Every other dayNothingUnrestricted

Almost all trials use the modified versions. True zero-calorie alternate-day fasting is rare in research because adherence collapses.

The comparative evidence

The most directly useful analysis compared alternate-day fasting, 5:2 and time-restricted eating head-to-head across trials [2]. Alternate-day fasting came out ahead. The BMJ systematic review of fasting strategies is broadly consistent [1].

One genuinely reassuring finding: a meta-analysis of appetite outcomes found intermittent fasting does not produce the compensatory hunger surge that critics predicted [3]. People do not reliably eat back the deficit on non-fasting days.

Weight loss efficacy

Where the evidence runs out

Cochrane examined intermittent fasting for cardiovascular disease prevention and concluded the evidence was insufficient [5] — not negative, insufficient. That is a different and more honest verdict than either enthusiasts or critics usually report. An earlier systematic review of fasting and weight loss reached similar conclusions about study quality [6].

The liver result

One of the more interesting positive findings: alternate-day fasting combined with aerobic exercise improved non-alcoholic fatty liver disease in a randomised trial, and the combination outperformed either component alone [7]. As with time-restricted eating, fasting plus exercise beats fasting.

Choosing between them

  • Pick on sustainability, not effect size. The largest effect in a trial is worthless if you stop in week six. Trial dropout rates are the most under-read number in this literature.
  • 5:2 is the usual compromise. Two hard days a week is tolerable for more people than every other day.
  • Fasting days need protein. A quarter of normal intake made of protein and vegetables is a very different day from a quarter made of toast.
  • Do not stack protocols. Combining alternate-day fasting with a compressed window on non-fasting days has not been tested and mostly produces a very large deficit with very poor adherence.

Common questions

Is 5:2 better than 16:8?
On average weight loss in comparative meta-analysis, yes [2]. On the probability that you are still doing it in a year, probably not.
Can I exercise on a fasting day?
Yes, though intensity often suffers. See training fasted.
Does it help fatty liver?
Alternate-day fasting plus exercise showed benefit in a randomised trial [7]. The exercise component appears to matter.
Do the two fasting days need to be consecutive?
Trials generally use non-consecutive days, and there is no evidence that consecutive is better.

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

  1. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials Semnani-Azad Z, Khan TA, Chiavaroli L, et al. · BMJ (Clinical research ed.) · 2025 · Meta-analysis DOIPubMed 40533200Full text
  2. A meta-analysis comparing the effectiveness of alternate day fasting, the 5:2 diet, and time-restricted eating for weight loss Elortegui Pascual P, Rolands MR, Eldridge AL, et al. · Obesity (Silver Spring, Md.) · 2023 · Meta-analysis DOIPubMed 36349432Full text
  3. The Effect of Intermittent Fasting on Appetite: A Systematic Review and Meta-Analysis Elsworth RL, Monge A, Perry R, et al. · Nutrients · 2023 · Meta-analysis DOIPubMed 37299567Full text
  4. Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials Patikorn C, Roubal K, Veettil SK, et al. · JAMA network open · 2021 · Systematic review DOIPubMed 34919135Full text
  5. Intermittent fasting for the prevention of cardiovascular disease Allaf M, Elghazaly H, Mohamed OG, et al. · The Cochrane database of systematic reviews · 2021 · Systematic review DOIPubMed 33512717Full text
  6. Intermittent fasting and weight loss: Systematic review Welton S, Minty R, O'Driscoll T, et al. · Canadian family physician Medecin de famille canadien · 2020 · Systematic review PubMed 32060194Full text
  7. Effect of alternate day fasting combined with aerobic exercise on non-alcoholic fatty liver disease: A randomized controlled trial Ezpeleta M, Gabel K, Cienfuegos S, et al. · Cell metabolism · 2023 · Randomised controlled trial DOIPubMed 36549296Full text
  8. Sustained alternate-day fasting potentiates doxorubicin cardiotoxicity Ozcan M, Guo Z, Valenzuela Ripoll C, et al. · Cell metabolism · 2023 · Randomised controlled trial DOIPubMed 36868222Full text
  9. Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans Stekovic S, Hofer SJ, Tripolt N, et al. · Cell metabolism · 2019 · Randomised controlled trial DOIPubMed 31471173
  10. Intermittent Fasting and Metabolic Health Vasim I, Majeed CN, DeBoer MD · Nutrients · 2022 · Review DOIPubMed 35276989Full text